GaN/AlGaN polar single quantum wells have attracted particular attention due to their large binding energies and the presence of strong internal electric fields that naturally induce long-lived excitons. In this case, the exciton transport is driven by the potential landscape caused by the local alteration of the built-in electric field, which results from the exciton accumulation at the same point. We study by spatially- and time-resolved PL experiments AlGaN/GaN coupled quantum wells (QW). The active zone of studied sample consists of a 30 nm thick Al0.20Ga0.80N barrier, a 4.4 nm (17 atomic monolayers) GaN QW followed by 1 monolayer of Al0.20Ga0.80N and a 1.6 nm (6 atomic monolayers) GaN QW and 30 nm thick Al0.20Ga0.80N cap layer. By solving Schroedinger-Poisson equation, we illustrate the two regimes of strongly screened electric field and the totally un-screened QW system. We show that the screening of the build-in electric field enhances the electron-hole wave-function overlap and increases the optical recombination energy. In other words, dipoledipole repulsion between excitons raises their potential energy and drastically increases their radiative recombination rate. A blue-shift of 0.3 eV under continues wave, localized excitation and a quasi-continuous red-shift of the emission as excitons are drifted away from the excitation spot up to distances of 90 μm are observed experimentally. Strong LO-phonon replica are also noticed, as a result of the strongly dipolar character of excitons [1]. These replicas follow the same position dependence as the zerophonon line. Using our Schroedinger-Poisson equation solver, we can extract from the measured blue-shift the position-dependent exciton density. Last, we present simulations of the observed transport features, namely the exciton densities and luminescence intensities by solving the nonlinear drift-diffusion equation. We establish that the long-range excitonic transport is mainly dominated by drift caused by repulsive dipole-dipole interaction between excitons in combination with their long lifetimes. [1] S. Kalliakos, X. B. Zhang, T. Taliercio, P. Lefebvre, B. Gil, N. Grandjean, B. Damilano, and J. Massies. Applied Physics Letters 80, 428 (2002).

We present the magnetoresistance (MR) of highly doped monolayer graphene layers grown by chemical vapor deposition on 6H-SiC. The magnetotransport studies are performed on a large temperature range, from T=1.7 K up to room temperature. The MR exhibits a maximum in the temperature range 120–240 K. The maximum is observed at intermediate magnetic fields (B=2–6 T), in between the weak localization and the Shubnikov-de Haas regimes. It results from the competition of two mechanisms. First, the low-field magnetoresistance increases continuously with T and has a purely classical origin. This positive MR is induced by thermal averaging and finds its physical origin in the energy dependence of the mobility around the Fermi energy. Second, the high-field negative MR originates from the electron-electron interaction (EEI). The transition from the diffusive to the ballistic regime is observed. The amplitude of the EEI correction points towards the coexistence of both long- and short-range disorder in these samples.

We present magnetotransport measurements on a high electron density AlGaN/GaN heterostructure with two subbands populated at $\tau$ = 1.6 K. The transport scattering times, $\tau_\text{tr}$ , of each subband are first derived at low magnetic field by taking into account the magneto-intersubband scattering term. Then the quantum scattering times, $\tau_\text{q}$, are extracted from independent Dingle plots, obtained at higher magnetic fields. All scattering times are studied as a function of the total electronic density, increased by the persistent photo-conductivity effect. A standard modelization, based on all common scattering mechanisms, reveals that the transport scattering time is governed by the short-range AlGaN/GaN interface roughness (IR) scattering, whereas the quantum scattering time is due to the smooth potential induced by remote ionized impurities (II) at the GaN surface. This intermediate situation of mixed disorder, where the $\tau_\text{tr}/\tau_\text{q}$ ratio is greater than one, does not indicate that the mobility is limited by Coulomb scattering. It is due to the unusual condition $\tau_\text{tr}^\text{II} >\!\!\!> \tau_\text{tr}^\text{IR} >\!\!\!> \tau_\text{q}^\text{II}$

The structural, optical, and transport properties of graphene grown by chemical vapor deposition (CVD) of propane under hydrogen on the Si face of SiC substrates have been investigated. We show that little changes in temperature during the growth can trigger the passivation of the SiC surface by hydrogen. Depending on the growth condition, hole or electron doping can be achieved, down to a few 1011 cm−2. When the growth temperature is high (T≈1500–1550∘C), we obtain electron-doped graphene monolayers lying on a buffer layer. When the growth temperature is slightly lowered (T≈1450–1500∘C), hole-doped graphene layers are obtained, lying on a hydrogen-passivated SiC surface, as confirmed by the enhancement of the mobility (of the order of 4500 cm2/Vs) and the persistence of weak localization almost up to room temperature (250 K). The high homogeneity of this graphene allows the observation of the half-integer quantum Hall effect, typical of graphene, at the centimeter scale in the best cases. The influence of the SiC steps on the transport properties is discussed.

Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. The challenges for the realization of long-range electron spin transport include rapid spin relaxation due to electron scattering. Scattering and, in turn, spin relaxation can be effectively suppressed in excitonic devices where the spin currents are carried by electrically neutral bosonic quasiparticles: excitons or exciton-polaritons. They can form coherent quantum liquids that carry spins over macroscopic distances. The price to pay is a finite lifetime of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems supporting bosonic spin transport, in particular to indirect excitons in coupled quantum wells. We describe the effect of spin-orbit interaction for the electron and the hole on the exciton spin, account for the Zeeman effect induced by external magnetic fields and long-range and short-range exchange splittings of the exciton resonances. We also consider exciton transport in the nonlinear regime and discuss the definitions of the exciton spin current, polarization current, and spin conductivity.